One Object won't Show up in Final Render even though it shows up in the Preview Render
Look at the camera icons on the outliner.
Click on the funnel icon on the top right and enable different filters.
When the camera icons are disabled, the object is disabled from rendering. To make it active click on the camera icon.

These icons are switches:
The Eye icon is to toggle object visibility on the 3D viewport.
The Arrow Icon toggles selectability.
The Camera Icon toggles rendering.
The Monitor icon disables the object in all viewports.
For more info refer to the blender manual
In older versions (2.79 or previous) the outliner the switches are here: